Seasonal cycles of currents, temperatures, winds, and sea level over the northeast Pacific continental shelf: 35°N to 48°N

Abstract

Seasonal cycles of coastal wind stress, adjusted sea level (ASL), shelf currents, and water temperatures off the west coast of North America (35°N to 48°N) are estimated by fitting annual and semiannual harmonics to data from 1981–1983. Longer records (9–34 years) of monthly ASL indicate that these two harmonics adequately represent the long‐term monthly average seasonal cycle and that the current measurement period is long enough to estimate the seasonal cycles. We characterize the differences between fall/winter and spring/summer as follows: For fall/winter, monthly mean winds north of 35°N are northward for 3–6 months (longer in the north than in the south); south of 35°N, the mean winds are near zero or weakly southward; monthly mean alongshore currents are northward over midshelf and shelf break at all locations sampled at depths of 35 m and deeper and are associated with high coastal sea levels and relatively warm water temperatures. For spring/summer, monthly mean wind stresses are southward at all latitudes for 3–6 months (longer in the south than in the north), sea levels are low, and water temperatures are relatively cool; monthly mean currents at 35 m depth over the shelf are southward for 1–6 months (longer at the shelf break than over midshelf and longer in the north than in the south), while the deeper currents are less southward or northward. The magnitudes of the seasonal cycles of all variables are maximum between approximately 38°N and 43°N, generally decreasing slightly to the north and greatly to the south. At each location the seasonal cycle of the alongshore current from 35 m depth at midshelf leads the sea level slightly and both lead the wind stress and temperatures by 1–2 months. The seasonal cycles of all variables show a south‐to‐north progression (south leads north by 1–2 months). At 48°N, annual mean currents at 50 m depth over the shelf break oppose the annual mean wind (northward wind and southward current). Similarly, at 35°N, annual mean currents at 35 m depth over both midshelf and shelf break are opposed to the annual mean wind (southward wind and northward current). From 35°N to 43°N, both summer and winter regimes are dominated by strongly fluctuating currents.